Drive schemes for cholesteric liquid crystal displays (ChLCD) are discussed in U.S. patent application Ser. No. 08/852,319, which is incorporated herein by reference. As discussed therein, a gray scale appearance for bistable cholesteric reflective displays is obtained by applying a voltage within a range of voltages during a selection phase, which is one of a series of phases for voltage application pulses, to obtain the desired gray scale appearance. In that disclosed drive scheme, it is only appreciated that the cholesteric material can be driven from a non-reflective focal conic texture to a reflective planar texture. Moreover, when the material is driven from a non-reflective state to a reflective state, no consideration is given to the initial state of the liquid crystal material. In other words, a wide range of voltages is applied to the material, no matter if the material was initially in the focal conic texture or in the twisted planar texture. Accordingly, a wide undefined range of voltage pulses is required to drive the liquid crystal material to obtain a gray scale appearance.
As discussed in U.S. patent application Ser. No. 08/852,319, time modulation of the selection phase voltage may be employed to control the level of gray scale reflectance of the liquid crystal material. However, it has been determined that this method of voltage application may not be suitable for some cholesteric liquid crystal materials.
An improvement of the foregoing method is disclosed in U.S. patent application Ser. No. 09/076,577, which is incorporated herein by reference. The '577 application is directed to a gray scale driving waveform that includes time modulating application of a portion of the waveform pulse in the form of a single bi-level pulse. This pulse includes a first voltage level for a first variable period of time and a second voltage, different than the first voltage level, for a second variable period of time. The sum of the first and second variable periods of time are equal to a set time period. Use of such a pulse is advantageous in that it allows for use of a lower frequency signal which, in turn, results in less power consumption by the display.
The above method has been found to be advantageous over the scheme disclosed in the patent to Wu, U.S. Pat. No. 5,933,203. The gray scale method described in the Wu patent uses a pulse number modulation technique that requires the use of higher frequency electric fields (waveforms or signals) for gray scale implementation. Due to the capacitive load of the cholesteric liquid crystal display, the higher frequency drive signals require significantly more power from the power source. However, the drive scheme disclosed in the '577 application, in combination with the capacitive load of the cholesteric liquid crystal display and the resistances of the electrodes and driver circuitry, causes the rising and falling edges of the waveforms to become “rounded” which lowers the magnitude or area integrated under the waveform outline. It will also be appreciated that the pixel bistable reflectance characteristics depend upon the magnitude of the waveform applied prior to removing the electric field. If the two drive signals, each applied to the electrodes of common cells, have the same amplitude to produce the same reflective characteristics, but the two signals have different drive frequencies, then the drive signal with the higher frequency needs to be applied to the corresponding cell (or pixels) for a longer duration than the lower frequency drive signal. Hence, the gray scale method described in the Wu patent will require a much longer image update duration than desired.
In light of the foregoing, it is evident that there is still a need in the art for drive schemes which more precisely drive cholesteric/chiral nematic liquid crystal material to an appropriate gray scale appearance by using less power. This is also a need for implementing such a drive scheme with either bipolar or unipolar waveforms.